Light-emitting device

ABSTRACT

In a light-emitting device, supply of current is controlled using a transistor having a normal gate electrode (a first gate electrode) and a second gate electrode for controlling threshold voltage. The light-emitting device comprises one or more switches for selecting conduction or non-conduction between the first gate electrode and a drain terminal of the transistor. When the threshold voltage of the transistor is acquired, the first gate electrode and the drain terminal of the transistor are brought into conduction with the switch, and the threshold voltage of the transistor is shifted by controlling the potential of the second gate electrode.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.13/612,035, filed Sep. 12, 2012, now allowed, which claims the benefitof a foreign priority application filed in Japan as Serial No.2011-200067 on Sep. 14, 2011, both of which are incorporated byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a light-emitting device in which atransistor is provided in each pixel.

2. Description of the Related Art

In an active matrix light-emitting device including light-emittingelements, in general, at least a light-emitting element, a transistor (aswitching transistor) that controls input of image signals to pixels,and a transistor (a driving transistor) that controls the value ofcurrent supplied to the light-emitting element in response to an imagesignal are provided in each pixel. In a light-emitting device having theabove structure, drain current of a driving transistor is supplied to alight-emitting element; thus, when the threshold voltage of drivingtransistors varies among pixels, the luminance of light-emittingelements varies correspondingly.

Patent Document 1 discloses a display device in which the thresholdvoltage of a TFT 6 (a driver element) is corrected so that variations inthreshold voltage do not influence the luminance of a light-emittingelement.

REFERENCE

Patent Document 1: Japanese Published Patent Application No. 2004-280059

In the display device disclosed in Patent Document 1, a gate electrodeand a drain electrode of the TFT 6 (the driver element) areshort-circuited when threshold voltage is detected; thus, the TFT 6operates in a saturation region. Thus, when current flowing from thedrain electrode of the TFT 6 to a source electrode of the TFT 6converges to 0 A, a potential difference between the gate electrode andthe source electrode equals the threshold voltage, so that the thresholdvoltage can be acquired.

In the display device disclosed in Patent Document 1, the gate electrodeand the drain electrode are short-circuited when the threshold voltageis detected; thus, the potential of the source electrode of the TFT 6does not exceed the potential of the gate electrode of the TFT 6. Inother words, the potential difference between the gate electrode and thesource electrode is not negative voltage. Thus, in the case where theTFT 6 is normally off and the threshold voltage of the TFT 6 is 0 V orhigher, the potential difference between the gate electrode and thesource electrode can equal the threshold voltage. In the case where theTFT 6 is normally on and the threshold voltage of the TFT 6 is negativevoltage, the potential difference between the gate electrode and thesource electrode cannot equal the threshold voltage. Consequently, whenthe TFT 6 is normally on, the threshold voltage cannot be acquired, andgeneration of unevenness in luminance of a light-emitting element due tovariations in threshold voltage cannot be prevented.

SUMMARY OF THE INVENTION

Under the technical background, it is an object of the present inventionto provide a light-emitting device in which variations in luminanceamong pixels due to variations in threshold voltage can be reduced evenwhen a transistor is normally on.

In a light-emitting device according to one embodiment of the presentinvention, supply of current to a light-emitting element is controlledusing a transistor having a normal gate electrode (a first gateelectrode) and a second gate electrode for controlling thresholdvoltage. The light-emitting device includes a switch for selectingconduction or non-conduction between the first gate electrode and adrain terminal of the transistor. When the threshold voltage of thetransistor is acquired, the gate electrode and the drain terminal of thetransistor are brought into conduction with the switch, and thethreshold voltage of the transistor is shifted by controlling thepotential of the second gate electrode.

The transistor for controlling supply of current to the light-emittingelement may be any insulated-gate field-effect transistor. Specifically,the transistor includes at least a first gate electrode, a second gateelectrode, a semiconductor film positioned between the first gateelectrode and the second gate electrode, a first insulating filmpositioned between the first gate electrode and the semiconductor film,and a second insulating film positioned between the second gateelectrode and the semiconductor film. The transistor may further includea source terminal and a drain terminal that are in contact with thesemiconductor film.

With the above structure, even when the transistor for controllingsupply of current to the light-emitting element is normally on, thetransistor can be normally off when the threshold voltage is acquired.Thus, while the first gate electrode and the drain terminal of thetransistor are brought into conduction with the switch, that is, areconnected to each other, a potential difference between the second gateelectrode and the source terminal can equal the threshold voltage.

In the light-emitting device according to one embodiment of the presentinvention, even when the transistor for controlling supply of current tothe light-emitting element is normally on, the threshold voltage can beacquired. Thus, the threshold voltage can be corrected, so thatvariations in luminance among pixels can be reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 illustrates the structure of a pixel included in a light-emittingdevice;

FIGS. 2A and 2B are enlarged views of a circuit 12;

FIG. 3 illustrates the structure of a pixel portion;

FIG. 4 illustrates the structure of a pixel included in a light-emittingdevice;

FIG. 5 is a timing chart illustrating the operation of the pixel;

FIGS. 6A and 6B schematically illustrate the operation of the pixel;

FIGS. 7A and 7B schematically illustrate the operation of the pixel;

FIGS. 8A and 8B each schematically illustrate a state where a capacitorand a light-emitting element are connected in series to each other;

FIG. 9 illustrates the structure of the pixel included in thelight-emitting device;

FIG. 10 illustrates the structure of a pixel included in alight-emitting device;

FIG. 11 is a top view of a pixel;

FIG. 12 is a cross-sectional view of the pixel;

FIG. 13 is a cross-sectional view of a pixel;

FIGS. 14A to 14C are cross-sectional views of pixels;

FIG. 15 is a perspective view of a panel; and

FIGS. 16A to 16E illustrate electronic devices.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention will be described in detail belowwith reference to the drawings. Note that the present invention is notlimited to the following description. It will be readily appreciated bythose skilled in the art that modes and details of the present inventioncan be modified in various ways without departing from the spirit andscope of the present invention. The present invention therefore shouldnot be construed as being limited to the following description of theembodiments.

Note that in this specification, the category of light-emitting devicesincludes panels in which a light-emitting element is formed in eachpixel, and modules in which ICs and the like including controllers aremounted on the panels.

Embodiment 1

FIG. 1 illustrates the structure of a pixel included in a light-emittingdevice according to one embodiment of the present invention. A pixel 10in FIG. 1 includes a switch 11, a circuit 12 for controlling the amountof current in response to an image signal, a switch 13, and alight-emitting element 14 supplied with the current from the circuit 12.

Specifically, the switch 11 controls whether an image signal supplied toa terminal 18 is supplied to the circuit 12. For example, the switch 11can be one or more transistors. Alternatively, the switch 11 may be acapacitor instead of one or more transistors.

The circuit 12 includes a transistor 15 whose drain current is suppliedto the light-emitting element 14, a switch 16, and a capacitor 17. Theswitch 16 selects conduction or non-conduction between a gate electrode(represented by G) and a drain terminal (represented by D) of thetransistor 15, that is, controls connection between the gate electrodeand the drain terminal of the transistor 15. The switch 16 can be one ormore transistors. The capacitor 17 holds a potential difference betweenthe gate electrode and a source terminal (represented by S) of thetransistor 15, that is, gate voltage Vgs. Note that the capacitor 17 isnot necessarily provided in the circuit 12 when gate capacitance formedbetween the gate electrode and an active layer of the transistor 15 issufficiently high, for example.

In one embodiment of the present invention, the transistor 15 includes aback gate electrode (a second gate electrode) for controlling thresholdvoltage in addition to a normal gate electrode (a first gate electrode).The potential of the gate electrode of the transistor 15 is controlledin response to an image signal supplied to the circuit 12 through theswitch 11. Further, the switch 13 controls supply of the potential of aterminal 21 to the back gate electrode (represented by BG). For example,the switch 13 can be one or more transistors. Alternatively, the switch13 may be a capacitor instead of one or more transistors.

Note that the terms “source terminal” and “drain terminal” of atransistor interchange with each other depending on the type of thechannel of the transistor or levels of potentials applied to electrodes.In general, in an n-channel transistor, an electrode to which a lowpotential is applied is called a source terminal, and an electrode towhich a high potential is applied is called a drain terminal. Further,in a p-channel transistor, an electrode to which a low potential isapplied is called a drain terminal, and an electrode to which a highpotential is applied is called a source terminal. In this specification,although the connection relation of the transistor is described assumingthat the source terminal and the drain terminal are fixed in some casesfor convenience, actually, the names of the source terminal and thedrain terminal interchange with each other depending on the relation ofthe potentials.

A “source terminal” of a transistor means a source region that is partof an active layer or a source electrode that is connected to an activelayer. Similarly, a “drain terminal” of a transistor means a drainregion that is part of an active layer or a drain electrode that isconnected to an active layer.

In this specification, the term “connection” means electrical connectionand corresponds to a state where current, voltage, or a potential can besupplied or transmitted. Accordingly, a connection state does not alwaysmean a direct connection state but includes an indirect connection statethrough an element such as a wiring, a conductive film, a resistor, adiode, or a transistor so that current, voltage, or a potential can besupplied or transmitted.

Even when independent components are connected to each other in acircuit diagram, there is the case where one conductive film hasfunctions of a plurality of components, such as the case where part of awiring functions as an electrode. The term “connection” in thisspecification also means such a case where one conductive film hasfunctions of a plurality of components.

In FIG. 1, the transistor 15 is an n-channel transistor. In that case,the source terminal of the transistor 15 is connected to an anode of thelight-emitting element 14. The drain terminal of the transistor 15 isconnected to a terminal 19, and a cathode of the light-emitting element14 is connected to a terminal 20. The potential of the terminal 19 ishigher than the sum of the potential of the terminal 20, thresholdvoltage Vthe of the light-emitting element 14, and threshold voltage Vthof the transistor 15. Thus, when the value of the drain current of thetransistor 15 is determined in response to an image signal supplied tothe circuit 12 through the switch 11, the light-emitting element 14emits light by supply of the drain current to the light-emitting element14.

In the case where the transistor 15 is a p-channel transistor, thesource terminal of the transistor 15 is connected to the cathode of thelight-emitting element 14. The drain terminal of the transistor 15 isconnected to the terminal 19, and the anode of the light-emittingelement 14 is connected to the terminal 20. The potential of theterminal 20 is higher than the sum of the potential of the terminal 19,the threshold voltage Vthe of the light-emitting element 14, and thethreshold voltage Vth of the transistor 15. As in the case where thetransistor 15 is an n-channel transistor, in the case where thetransistor 15 is a p-channel transistor, when the value of the draincurrent of the transistor 15 is determined in response to an imagesignal supplied to the circuit 12 through the switch 11, thelight-emitting element 14 emits light by supply of the drain current tothe light-emitting element 14.

In one embodiment of the present invention, before the value of thedrain current of the transistor 15 is determined in response to an imagesignal, the threshold voltage of the transistor 15 is acquired while thegate electrode and the drain terminal of the transistor 15 are broughtinto conduction with the switch 16. By determining the value of thedrain current of the transistor 15 in response to an image signal afterthe threshold voltage is acquired, variations in threshold voltage amongpixels can be prevented from influencing the value of the drain current.

In one embodiment of the present invention, the transistor 15 includesthe back gate electrode for controlling the threshold voltage inaddition to the normal gate electrode, as described above. The thresholdvoltage Vth of the transistor 15 is controlled in response to apotential applied to the back gate electrode. In one embodiment of thepresent invention, in the case of the normally-on transistor 15, whenthe threshold voltage is acquired, by controlling the potential of theback gate electrode, the threshold voltage Vth of the transistor 15 isshifted so that the transistor 15 is normally off. The shift amount ofthe threshold voltage Vth of the transistor 15 is controlled in responseto the level of the potential of the back gate electrode, specifically,a potential difference between the source terminal and the back gateelectrode.

Specifically, in the case where the transistor 15 is an n-channeltransistor, the transistor 15 is normally on when the threshold voltageVth of the transistor 15 is negative voltage. Thus, in the case wherethe transistor 15 is an n-channel transistor, the threshold voltage Vthis shifted in a positive direction by setting the potential of the backgate electrode lower than the potential of the source terminal, so thatthe transistor 15 is normally off. In the case where the transistor 15is a p-channel transistor, the transistor 15 is normally on when thethreshold voltage Vth of the transistor 15 is positive voltage. Thus, inthe case where the transistor 15 is a p-channel transistor, thethreshold voltage Vth is shifted in a negative direction by setting thepotential of the back gate electrode higher than the potential of thesource terminal, so that the transistor 15 is normally off.

Note that it is difficult to acquire the threshold voltage when thetransistor 15 is normally on. The reason is described below using anexample in which the transistor 15 is an n-channel transistor.

FIGS. 2A and 2B are enlarged views of the circuit 12. As illustrated inFIG. 2A, in the case where the transistor 15 is an n-channel transistor,before the threshold voltage is acquired, the terminal 19 is kept at apotential that is higher than the potential of the source terminal ofthe transistor 15. Specifically, a potential difference Von is producedbetween the source terminal of the transistor 15 and the terminal 19 sothat the potential of the terminal 19 is higher than the sum of thepotential of the source terminal of the transistor 15 and the thresholdvoltage Vth of the transistor 15. Then, when the switch 16 is turned on,the gate electrode and the drain terminal of the transistor 15 areconnected to each other.

Thus, as illustrated in FIG. 2A, the gate voltage Vgs of the transistor15 equals the potential difference Von, so that the transistor 15 isturned on and drain current flows. Under the above condition, the gateelectrode of the transistor 15 is connected to one electrode of thecapacitor 17 and the source terminal of the transistor 15 is connectedto the other electrode of the capacitor 17, so that the drain current ofthe transistor 15 flows only to the capacitor 17.

With the above structure, electric charge accumulated in the capacitor17 is released, so that the potential of the source terminal of thetransistor 15 is raised. Then, the rise in potential of the sourceterminal leads to a gradual decrease in the gate voltage Vgs of thetransistor 15, though the gate voltage Vgs of the transistor 15 equalsthe potential difference Von at the beginning of supply of the draincurrent.

In the case where the transistor 15 is normally off, as the gate voltageVgs approaches the threshold voltage Vth, the drain current becomes 0 A.Accordingly, the threshold voltage Vth is held in the capacitor 17, andthe acquisition of the threshold voltage Vth is terminated. In the casewhere the transistor 15 is normally on, the threshold voltage Vth isnegative voltage. Thus, in order to acquire the threshold voltage Vth,the potential of the gate electrode should be lower than the potentialof the source terminal, and the gate voltage Vgs should be negativevoltage. However, since the terminal 19 is kept at the potential that ishigher than the potential of the source terminal of the transistor 15 asdescribed above, the potential of the gate electrode does not fall belowthe potential of the source terminal. Thus, in the case where thetransistor 15 is normally on, as illustrated in FIG. 2B, as the gatevoltage Vgs approaches 0 V and a potential difference between the sourceterminal and the drain terminal of the transistor 15 approaches 0 V, thedrain current of the transistor 15 becomes 0 A. Accordingly, thethreshold voltage Vth is not held in the capacitor 17.

In one embodiment of the present invention, even in the case where thetransistor 15 is normally on, when the threshold voltage Vth isacquired, the transistor 15 is normally off by shifting the thresholdvoltage Vth. By acquiring the threshold voltage, variations in thresholdvoltage among pixels can be corrected, so that variations in luminanceamong the pixels can be reduced.

Further, as described above, in one embodiment of the present invention,any structure can be used as long as connection between the gateelectrode and the drain terminal of the transistor 15 can be controlledwith the switch 16. Any structure can be used as long as the gatevoltage Vgs of the transistor 15 can be held in the capacitor 17 or thegate capacitance of the transistor 15 in the case where the capacitor 17is not provided. Alternatively, any structure can be used as long aselectric charge accumulated in the capacitor 17 is released by draincurrent flowing to the transistor 15 and thus the threshold voltage ofthe transistor 15 is held in the capacitor 17. Thus, the circuit 12 mayinclude a circuit component such as a transistor, a capacitor, aresistor, or an inductor in addition to the transistor 15, the switch16, and the capacitor 17. Furthermore, in order to achieve thestructure, a different circuit component may be provided among thetransistor 15, the switch 16, the capacitor 17, and the terminal 19.

FIG. 3 illustrates a structure example of a pixel portion in alight-emitting device according to one embodiment of the presentinvention. In FIG. 3, a pixel portion 40 includes a plurality of pixels10 arranged in a matrix. The pixel portion 40 includes at least scanlines GL for selecting the plurality of pixels 10 row by row and signallines SL for transmitting image signals to the selected pixels 10. Eachof the plurality of pixels 10 is connected to at least one of the scanlines GL and at least one of the signal lines SL.

Note that the kinds and number of the lines can be determined by thestructure, number, and position of the pixels 10. Specifically, in thecase of the pixel portion 40 in FIG. 3, the pixels 10 are arranged in amatrix of x columns×y rows, and signal lines SL1 to SLx and scan linesGL1 to GLy are provided in the pixel portion 40.

In one embodiment of the present invention, even when the pixel portion40 includes the pixel 10 having the normally-on transistor 15 and thepixel 10 having the normally-off transistor 15, the transistors 15 arenormally off in all the pixels 10, so that the threshold voltage can beacquired.

Specifically, in the pixel portion 40 in FIG. 3, one potential forcorrecting the threshold voltage may be applied to the back gateelectrodes of the transistors 15 in all the pixels 10. Alternatively,one potential for correcting the threshold voltage may be applied to theback gate electrodes of the transistors 15 in pixels of the same row,that is, a plurality of pixels connected to the same scan line GL.Alternatively, one potential for correcting the threshold voltage may beapplied to the back gate electrodes of the transistors 15 in pixels ofthe same column, that is, a plurality of pixels connected to the samesignal line SL. Accordingly, even when the transistors 15 are normallyon in all the pixels 10 in the pixel portion 40 or the pixel portion 40includes the pixel 10 having the normally-on transistor 15 and the pixel10 having the normally-off transistor 15, the threshold voltage can beacquired.

Embodiment 2

FIG. 4 illustrates a specific structure example of a pixel in alight-emitting device according to one embodiment of the presentinvention.

As in FIG. 1, the pixel 10 in FIG. 4 includes the switch 11, the circuit12, the switch 13, and the light-emitting element 14. In the pixel 10 inFIG. 4, the switch 11 is a transistor 30. Further, the circuit 12includes the transistor 15, the capacitor 17, transistors 31 to 36, anda capacitor 38. The transistor 31 corresponds to the switch 16 inFIG. 1. The switch 13 is a transistor 37.

Note that in FIG. 4, the transistor 15 is an n-channel transistor.

Specifically, in the pixel 10 in FIG. 4, a gate electrode of thetransistor 30 is connected to a scan line GLa. One of a source terminaland a drain terminal of the transistor 30 is connected to the signalline SL. The other of the source terminal and the drain terminal of thetransistor 30 is connected to the gate electrode of the transistor 15. Agate electrode of the transistor 31 is connected to the scan line GLa.One of a source terminal and a drain terminal of the transistor 31 isconnected to the gate electrode of the transistor 15. The other of thesource terminal and the drain terminal of the transistor 31 is connectedto the drain terminal of the transistor 15. A gate electrode of thetransistor 32 is connected to the scan line GLa. One of a sourceterminal and a drain terminal of the transistor 32 is connected to oneelectrode of the capacitor 17. The other of the source terminal and thedrain terminal of the transistor 32 is connected to a line VLa. A gateelectrode of the transistor 33 is connected to a scan line GLb. One of asource terminal and a drain terminal of the transistor 33 is connectedto the drain terminal of the transistor 15. The other of the sourceterminal and the drain terminal of the transistor 33 is connected to theline VLa. A gate electrode of the transistor 34 is connected to the scanline GLb. One of a source terminal and a drain terminal of thetransistor 34 is connected to the gate electrode of the transistor 15.The other of the source terminal and the drain terminal of thetransistor 34 is connected to one electrode of the capacitor 17. A gateelectrode of the transistor 35 is connected to the scan line GLb. One ofa source terminal and a drain terminal of the transistor 35 is connectedto the source terminal of the transistor 15 and the other electrode ofthe capacitor 17. The other of the source terminal and the drainterminal of the transistor 35 is connected to the anode of thelight-emitting element 14. A gate electrode of the transistor 36 isconnected to a scan line GLc. One of a source terminal and a drainterminal of the transistor 36 is connected to the other electrode of thecapacitor 17 and the source terminal of the transistor 15. The other ofthe source terminal and the drain terminal of the transistor 36 isconnected to a line VLb. One electrode of the capacitor 38 is connectedto the back gate electrode of the transistor 15. The other electrode ofthe capacitor 38 is connected to the source terminal of the transistor15. A gate electrode of the transistor 37 is connected to the scan lineGLc. One of a source terminal and a drain terminal of the transistor 37is connected to the back gate electrode of the transistor 15. The otherof the source terminal and the drain terminal of the transistor 37 isconnected to a line VLc.

Note that in the pixel 10 in FIG. 4, one of the source terminal and thedrain terminal of the transistor 31 is connected to the other of thesource terminal and the drain terminal of the transistor 30, and theother of the source terminal and the drain terminal of the transistor 31is connected to one of the source terminal and the drain terminal of thetransistor 33. The connection of the transistor 31 may be any connectionas long as the transistor 31 can control the connection between the gateelectrode and the drain terminal of the transistor 15. Thus, forexample, as in the pixel 10 in FIG. 9, one of the source terminal andthe drain terminal of the transistor 31 may be connected to the other ofthe source terminal and the drain terminal of the transistor 30 and oneof the source terminal and the drain terminal of the transistor 33, andthe other of the source terminal and the drain terminal of thetransistor 31 may be connected to one of the source terminal and thedrain terminal of the transistor 34.

Next, the operation of the pixel 10 in FIG. 4 is described.

FIG. 5 is an example of a timing chart of a potential Vdata applied tothe signal line SL and potentials applied to the scan line GLa, the scanline GLb, and the scan line GLc, respectively. Note that in the timingchart in FIG. 5, the transistor 15 and the transistors 30 to 37 are alln-channel transistors.

The operation of the pixel 10 can be described with four separateperiods t1 to t4, as illustrated in FIG. 5. FIGS. 6A and 6B and FIGS. 7Aand 7B schematically illustrate the operation of the pixel 10 in eachperiod. Note that in FIGS. 6A and 6B and FIGS. 7A and 7B, thetransistors 30 to 37 serving as switching elements are illustrated asswitches.

Through the periods t1 to t4, a potential Vano is applied to the lineVLa, a potential V0 is applied to the line VLb, a potential V1 isapplied to the line VLc, and a potential Vcat is applied to the cathodeof the light-emitting element 14. A difference between the potentialVano and the potential Vcat in the case of the potential Vcat used as areference is higher than the threshold voltage Vthe of thelight-emitting element 14. Note that the threshold voltage Vthe of thelight-emitting element 14 is assumed to be 0 V in the followingdescription.

First, as illustrated in FIG. 5, in the period t1, low potentials areapplied to the scan lines GLa and GLb, and a high potential is appliedto the scan line GLc. Thus, the transistors 36 and 37 are turned on, andthe transistors 30 to 35 are turned off.

FIG. 6A schematically illustrates the operation of the pixel 10 in theperiod t1. In the period t1, the transistors 36 and 37 are turned on andthe transistors 30 to 35 are turned off as described above, so that thepotential V1 is applied to the back gate electrode of the transistor 15,and the potential V0 is applied to the source terminal of the transistor15. Thus, a potential difference between the back gate electrode and thesource terminal becomes V1−V0 and is held in the capacitor 38.

Note that in this embodiment, V1−V0 is negative voltage. When thepotential difference between the back gate electrode and the sourceterminal of the transistor 15 becomes V1−V0, the threshold voltage Vthof the transistor 15 is shifted in a positive direction. Thus, even whenthe transistor 15 is normally on when the potential difference betweenthe back gate electrode and the source terminal is 0 V, the thresholdvoltage Vth is shifted in a positive direction to be 0 V or higher.Consequently, the transistor 15 can be normally off.

Next, as illustrated in FIG. 5, in the period t2, a high potential isapplied to the scan line GLa, and low potentials are applied to the scanlines GLb and GLc. Thus, the transistors 30 to 32 are turned on, and thetransistors 33 to 37 are turned off. The potential Vdata of an imagesignal is applied to the signal line SL.

FIG. 6B schematically illustrates the operation of the pixel 10 in theperiod t2. At the beginning of the period t2, the transistors 30 to 32are turned on and the transistors 33 to 37 are turned off as describedabove and the potential Vdata of the image signal is applied to thesignal line SL, so that the potential of the source terminal of thetransistor 15 and the other electrode of the capacitor 17 (the potentialof a node A) becomes the potential V0. Further, the potential of oneelectrode of the capacitor 17 (the potential of a node B) becomes thepotential Vano. Thus, a potential difference applied to the capacitor 17becomes Vano−V0.

In addition, the potential difference V1−V0 between the back gateelectrode and the source terminal of the transistor 15 is held in thecapacitor 38. Thus, the threshold voltage Vth of the transistor 15remains at 0 V or higher, and the transistor 15 is kept normally off.

Further, the potential of the gate electrode of the transistor 15 (thepotential of a node C) becomes the potential Vdata. Thus, the gatevoltage Vgs of the transistor 15 becomes Vdata−V0. Naturally, the levelof the potential Vdata of the image signal depends on image datacontained in the image signal; however, the potential Vdata is higherthan the sum of the potential V0 and the threshold voltage Vth of thetransistor 15. Consequently, the transistor 15 is turned on, andelectric charge accumulated in the capacitor 17 is released via thetransistor 15.

After the electric charge is released from the capacitor 17, thepotential of the source terminal of the transistor 15 is raised, and thegate voltage Vgs that is the potential difference Vdata−V0 at thebeginning of the period t2 approaches the threshold voltage Vth overtime. In addition, although the potential difference applied to thecapacitor 17 at the beginning of the period t2 is Vano−V0, after theelectric charge is released from the capacitor 17, the potentialdifference held in the capacitor 17 approaches Vano−Vdata+Vth over time.Consequently, the transistor 15 is eventually turned off.

Thus, in one embodiment of the present invention, even when thetransistor 15 is normally on, the transistor 15 is normally off byshifting the threshold voltage Vth of the transistor 15 in the periodt1, so that the threshold voltage Vth of the transistor 15 can beacquired in the period t2.

Note that in one embodiment of the present invention, the period t2 isnot necessarily terminated when the gate voltage Vgs of the transistor15 equals the threshold voltage Vth. For example, when the gate voltageVgs of the transistor 15 is lower than the potential difference Vdata−V0and higher than the threshold voltage Vth, the period t2 may beterminated.

Next, as illustrated in FIG. 5, in the period t3, low potentials areapplied to the scan lines GLa, GLb, and GLc. Thus, the transistors 30 to37 are turned off.

FIG. 7A schematically illustrates the operation of the pixel 10 in theperiod t3. The transistors 30 to 37 are turned off as described above,so that a potential difference Vano−Vdata+Vth is held in the capacitor17. In addition, the potential difference V1−V0 between the back gateelectrode and the source terminal of the transistor 15 is held in thecapacitor 38.

Next, as illustrated in FIG. 5, in the period t4, a high potential isapplied to the scan line GLb, and low potentials are applied to the scanlines GLa and GLc. Thus, the transistors 33 to 35 are turned on, and thetransistors 30, 31, 32, 36, and 37 are turned off.

FIG. 7B schematically illustrates the operation of the pixel 10 in theperiod t4. The transistors 33 to 35 are turned on and the transistors30, 31, 32, 36, and 37 are turned off in the period t4 as describedabove, so that the potential difference V1−V0 between the back gateelectrode and the source terminal of the transistor 15 is held in thecapacitor 38. Thus, the threshold voltage Vth of the transistor 15remains at 0 V or higher, and the transistor 15 is kept normally off.

Ideally, the potential difference Vano−Vdata+Vth held in the capacitor17 is applied between the gate electrode and the source terminal of thetransistor 15 as the gate voltage Vgs of the transistor 15.

Note that the gate voltage Vgs of the transistor 15 actually depends onthe ratio between the capacitance of the capacitor 17 and thecapacitance of the light-emitting element 14; thus, the gate voltage Vgsof the transistor 15 is not necessarily ideal voltage, that is, thepotential difference Vano−Vdata+Vth. A potential VA of the node A in theperiod t4 is described in detail below.

FIG. 8A is a circuit diagram of the capacitor 17. The capacitor 17 hascapacitance C1. As illustrated in FIG. 8A, at the termination of theperiod t3, the potential of one electrode of the capacitor 17(corresponding to the node B) is the potential Vano, and the potentialof the other electrode of the capacitor 17 (corresponding to the node A)is a potential Vdata−Vth. Thus, the potential difference Vano−Vdata+Vthis held in the capacitor 17.

In the period t4, the capacitor 17 and the light-emitting element 14 areconnected in series to each other through the transistor 35. FIG. 8Bschematically illustrates a state where the capacitor 17 and thelight-emitting element 14 are connected in series to each other. In FIG.8B, the light-emitting element 14 is illustrated as one capacitor. Thelight-emitting element 14 has capacitance C2. As illustrated in FIG. 8B,at the termination of the period t4, the potential of the node B is thepotential Vano, and the potential of the cathode of the light-emittingelement 14 is the potential Vcat. The potential of the other electrodeof the capacitor 17 and the anode of the light-emitting element 14(corresponding to the node A) is the potential VA.

The potential VA of the node A depends on the ratio between thecapacitance C1 of the capacitor 17 and the capacitance C2 of thelight-emitting element 14. Specifically, in the case where the potentialof the node A at the termination of the period t4 is the potential VA,the gate voltage Vgs of the transistor 15 in the period t4 isrepresented by Equation 1. Note that Equation 1 shows the case where thepotential of the node A is the potential Vdata−Vth in the period t3.

Vgs=Vano−VA=C2(Vano−Vdata)/(C1+C2)+Vth  (Equation 1)

Note that ideal gate voltage Vgs at the termination of the period t4equals Vano−Vdata+Vth. When the gate voltage Vgs has this value, evenwhen the threshold voltage Vth of the transistors 15 varies, thevariation does not influence the drain current of the transistors 15. Inorder that the gate voltage Vgs approaches the ideal voltage, fromEquation 1, it is clear that C2/(C1+C2) preferably approaches 1. Inother words, when the capacitance C2 of the light-emitting element 14 ismuch higher than the capacitance C1 of the capacitor 17, the gatevoltage Vgs can approach the ideal voltage.

If the gate voltage Vgs is close to Vano−Vdata+Vth in the period t4, thethreshold voltage Vth is added to the gate voltage Vgs of the transistor15. Consequently, variations in the threshold voltage Vth of thetransistors 15 can be prevented from influencing the value of the draincurrent supplied to the light-emitting element 14. Alternatively, evenwhen the transistor 15 is degraded and the threshold voltage Vth ischanged, the change in the threshold voltage Vth can be prevented frominfluencing the value of the drain current supplied to thelight-emitting element 14. Thus, it is possible to provide alight-emitting device capable of reducing luminance unevenness anddisplaying high-quality images.

The period t3 is not necessarily provided. The period t4 may be startedright after the period t2. Note that when the period t3 is provided, apotential applied to the scan line GLb can be changed from a low levelto a high level after a potential applied to the scan line GLa ischanged from a high level to a low level. Consequently, the potentialdifference Vano−Vdata+Vth held in the capacitor 17 can be prevented fromvarying due to the change in potential applied to the scan line GLb.

The above operation is performed on the pixels 10 row by row. Imagesignals are written to all the pixels 10 in the pixel portion row byrow, so that images are displayed.

In the light-emitting device according to one embodiment of the presentinvention, for example, in the case where amorphous silicon or an oxidesemiconductor is used for a semiconductor film of the transistor 15,even when the transistor 15 is normally on, luminance unevenness can bereduced and high-quality images can be displayed.

Note that in the case where the gate voltage Vgs at the termination ofthe period t2 is not equal to the threshold voltage Vth but is lowerthan the potential difference Vdata−V0 and higher than the thresholdvoltage Vth as described above, not only the variations in the thresholdvoltage of the transistors 15 but also variations in mobility can beprevented from influencing the luminance of the light-emitting element14. This is described in detail below.

Current Id that flows to the light-emitting element 14 is represented bykμ(Vgs−Vth)²/2, where μ is the mobility of the transistor 15 and k is aconstant that depends on the channel length, channel width, and gatecapacitance of the transistor 15. In the case where the mobility μ isnot corrected, the drain current Id that flows to the light-emittingelement 14 increases as the mobility μ increases, whereas the draincurrent Id that flows to the light-emitting element 14 decreases as themobility μ decreases.

For example, in the case where the potential of the node A at thetermination of the period t2 is lower than Vdata−Vth, the gate voltageVgs of the transistor 15 is voltage Va. The voltage Va is the sum of thethreshold voltage Vth and offset voltage Vb. In that case, at thetermination of the period t2, a potential difference Vano−Vdata+Vb+Vthis held in the capacitor 17.

Then, in the period t4, the potential difference held in the capacitor17 is the gate voltage Vgs of the transistor 15. Thus, the drain currentId in the period t4 is represented by kμ(Vano−Vdata+Vb)²/2.Consequently, even in the case where the potential of the node A at thetermination of the period t2 is lower than Vdata−Vth, a change in valueof drain current due to variations in the threshold voltage Vth iscanceled.

In the case where the transistor 15 is an n-channel transistor, theoffset voltage Vb is positive voltage. Accordingly, as the mobility μdecreases, the absolute value of the drain current Id increases. Incontrast, as the mobility μ increases, the absolute value of the draincurrent Id decreases. Thus, Vb serves as a correction term forcorrecting variations in the drain current Id due to the mobility μ inthe period t4; a decrease in the drain current Id can be inhibited evenwhen the mobility μ decreases, whereas an increase in the drain currentId can be inhibited even when the mobility μ increases.

This embodiment can be combined with any of the other embodiments asappropriate.

Embodiment 3

FIG. 10 illustrates a specific structure example of a pixel in alight-emitting device according to one embodiment of the presentinvention.

As in FIG. 1, the pixel 10 in FIG. 10 includes the switch 11, thecircuit 12, the switch 13, and the light-emitting element 14. In thepixel 10 in FIG. 10, the switch 11 is a transistor 51. Further, thecircuit 12 includes the transistor 15, the capacitor 17, transistors 52to 55, and capacitors 57 and 58. The transistor 52 corresponds to theswitch 16 in FIG. 1. The switch 13 is a transistor 56.

Note that in FIG. 10, the transistor 15 is an n-channel transistor.

Specifically, in the pixel 10 in FIG. 10, a gate electrode of thetransistor 51 is connected to the scan line GLa. One of a sourceterminal and a drain terminal of the transistor 51 is connected to thesignal line SL. The other of the source terminal and the drain terminalof the transistor 51 is connected to the gate electrode of thetransistor 15 and one electrode of the capacitor 17. A gate electrode ofthe transistor 52 is connected to the scan line GLb. One of a sourceterminal and a drain terminal of the transistor 52 is connected to thegate electrode of the transistor 15 and one electrode of the capacitor17. The other of the source terminal and the drain terminal of thetransistor 52 is connected to the drain terminal of the transistor 15. Agate electrode of the transistor 53 is connected to the scan line GLb.One of a source terminal and a drain terminal of the transistor 53 isconnected to the line VLb. The other of the source terminal and thedrain terminal of the transistor 53 is connected to the drain terminalof the transistor 15. A gate electrode of the transistor 54 is connectedto the scan line GLc. One of a source terminal and a drain terminal ofthe transistor 54 is connected to the drain terminal of the transistor15. The other of the source terminal and the drain terminal of thetransistor 54 is connected to the line VLa. A gate electrode of thetransistor 55 is connected to a scan line GLd. One of a source terminaland a drain terminal of the transistor 55 is connected to the anode ofthe light-emitting element 14. The other of the source terminal and thedrain terminal of the transistor 55 is connected to a line VLd. A gateelectrode of the transistor 56 is connected to a scan line GLe. One of asource terminal and a drain terminal of the transistor 56 is connectedto the back gate electrode of the transistor 15. The other of the sourceterminal and the drain terminal of the transistor 56 is connected to theline VLc. One electrode of the capacitor 57 is connected to the sourceterminal of the transistor 15, the other electrode of the capacitor 17,and the anode of the light-emitting element 14. The other electrode ofthe capacitor 57 is connected to the line VLd. One electrode of thecapacitor 58 is connected to the back gate electrode of the transistor15. The other electrode of the capacitor 58 is connected to the sourceterminal of the transistor 15.

Next, the operation of the pixel 10 in FIG. 10 is described.

The operation of the pixel 10 in FIG. 10 can be described with fiveseparate periods t1 to t5.

Through the periods t1 to t5, the potential Vano is applied to the lineVLa, a potential V2 is applied to the line VLb, a potential V3 isapplied to the line VLc, a potential V4 is applied to the line VLd, andthe potential Vcat is applied to the cathode of the light-emittingelement 14. The difference between the potential Vano and the potentialVcat in the case of the potential Vcat used as a reference is higherthan the threshold voltage Vthe of the light-emitting element 14. Notethat the threshold voltage Vthe of the light-emitting element 14 isassumed to be 0 V in the following description. The potential V2 ishigher than the potential Vcat and lower than the potential Vano. Thepotential V3 is lower than the potential Vcat and the potential V4. Thepotential V4 is lower than the potential Vcat.

First, in the period t1, the transistors 55 and 56 are turned on, andthe transistors 51 to 54 are turned off. Thus, in the period t1, thepotential V3 is applied to the back gate electrode of the transistor 15,and the potential V4 is applied to the source terminal of the transistor15. Consequently, the potential difference between the back gateelectrode and the source terminal becomes V3−V4 and is held in thecapacitor 58.

Since the potential difference V3−V4 between the back gate electrode andthe source terminal of the transistor 15 is negative voltage, thethreshold voltage Vth of the transistor 15 is shifted in a positivedirection. Thus, even when the transistor 15 is normally on when thepotential difference between the back gate electrode and the sourceterminal is 0 V, the threshold voltage Vth is shifted in a positivedirection to be 0 V or higher. Consequently, the transistor 15 can benormally off.

Next, in the period t2, the transistors 52, 53, and 55 are turned on,and the transistors 51, 54, and 56 are turned off. The potentialdifference V3−V4 between the back gate electrode and the source terminalof the transistor 15 is held in the capacitor 58. Thus, the thresholdvoltage Vth of the transistor 15 remains at 0 V or higher, and thetransistor 15 is kept normally off. Further, in the period t2, the gatevoltage Vgs of the transistor 15 becomes a potential difference V2−V4.Consequently, the transistor 15 is turned on, and drain current flows tothe transistor 15.

Next, in the period t3, the transistors 52 and 53 are turned on, and thetransistors 51, 54, 55, and 56 are turned off. The potential differenceV3−V4 between the back gate electrode and the source terminal of thetransistor 15 is held in the capacitor 58. Thus, the threshold voltageVth of the transistor 15 remains at 0 V or higher, and the transistor 15is kept normally off. After electric charge accumulated in the capacitor17 is released through the transistor 15, the potential of the sourceterminal of the transistor 15 is raised. In addition, the gate voltageVgs that is the potential difference V2−V4 at the beginning of theperiod t3 approaches the threshold voltage Vth over time. Consequently,the transistor 15 is eventually turned off.

Thus, in one embodiment of the present invention, even when thetransistor 15 is normally on, the transistor 15 is normally off byshifting the threshold voltage Vth of the transistor 15 in the periodt1, so that the threshold voltage Vth of the transistor 15 can beacquired in the period t3.

Note that in one embodiment of the present invention, the period t3 isnot necessarily terminated when the gate voltage Vgs of the transistor15 equals the threshold voltage Vth. For example, when the gate voltageVgs of the transistor 15 is lower than the potential difference V2−V4and higher than the threshold voltage Vth, the period t3 may beterminated. Accordingly, as in the pixel 10 in FIG. 4, not onlyvariations in the threshold voltage Vth but also variations in themobility of the transistors 15 can be corrected.

Next, in the period t4, the transistor 51 is turned on, and thetransistors 52 to 56 are turned off. The potential Vdata of an imagesignal is applied to the signal line SL. The potential difference V3−V4between the back gate electrode and the source terminal of thetransistor 15 is held in the capacitor 58. Thus, the threshold voltageVth of the transistor 15 remains at 0 V or higher, and the transistor 15is kept normally off. When the potential Vdata is applied to the gateelectrode of the transistor 15, the gate voltage Vgs of the transistor15 is ideally a potential difference Vdata−Vcat+Vth.

Note that the gate voltage Vgs of the transistor 15 actually depends onthe ratio between the capacitance of the capacitor 17 and thecapacitance of the capacitor 57 and the light-emitting element 14; thus,the gate voltage Vgs of the transistor 15 is not necessarily idealvoltage, that is, the potential difference Vdata−Vcat+Vth. However, asin the pixel 10 in FIG. 4, when the total capacitance of the capacitor57 and the light-emitting element 14 is much higher than the capacitanceof the capacitor 17, the gate voltage Vgs can approach the idealvoltage.

Next, in the period t5, the transistor 54 is turned on, and thetransistors 51, 52, 53, 55, and 56 are turned off. The potentialdifference V3−V4 between the back gate electrode and the source terminalof the transistor 15 is held in the capacitor 58. Thus, the thresholdvoltage Vth of the transistor 15 remains at 0 V or higher, and thetransistor 15 is kept normally off.

The transistor 15 supplies drain current based on the gate voltage Vgsof the transistor 15 to the light-emitting element 14. The luminance ofthe light-emitting element 14 depends on the value of the drain current.As the drain current increases, the luminance of the light-emittingelement 14 increases. As the drain current decreases, the luminance ofthe light-emitting element 14 decreases.

If the gate voltage Vgs is close to Vdata−Vcat+Vth in the period t4, thethreshold voltage Vth is added to the gate voltage Vgs of the transistor15. Consequently, in the period t5, variations in the threshold voltageVth of the transistors 15 can be prevented from influencing the value ofthe drain current supplied to the light-emitting element 14.Alternatively, even when the transistor 15 is degraded and the thresholdvoltage Vth is changed, the change in the threshold voltage Vth can beprevented from influencing the value of the drain current supplied tothe light-emitting element 14. Thus, it is possible to provide alight-emitting device capable of reducing luminance unevenness anddisplaying high-quality images.

The above operation is performed on the pixels 10 row by row. Imagesignals are written to all the pixels 10 in the pixel portion row byrow, so that images are displayed.

In the light-emitting device according to one embodiment of the presentinvention, for example, in the case where amorphous silicon or an oxidesemiconductor is used for the semiconductor film of the transistor 15,even when the transistor 15 is normally on, luminance unevenness can bereduced and high-quality images can be displayed.

This embodiment can be combined with any of the other embodiments asappropriate.

Embodiment 4

The pixel layout of a light-emitting device according to one embodimentof the present invention is described with reference to FIG. 11 and FIG.12 by using the pixel 10 in FIG. 4 as an example. FIG. 11 is an exampleof a top view of the pixel. FIG. 12 is an example of a cross-sectionalview taken along dashed lines A1-A2 and A3-A4 in the top view in FIG.11. Note that in order to clearly show the pixel layout, a variety ofinsulating films are not illustrated in the top view of the pixel inFIG. 11. In addition, in order to clearly show the layout of a varietyof semiconductor elements included in the pixel, an EL layer and acathode are not illustrated in the top view of the pixel in FIG. 11.

In the pixel in FIG. 11 and FIG. 12, the transistor 30 includes, over asubstrate 800 having an insulating surface, a conductive film 801functioning as a gate electrode, a gate insulating film 802 over theconductive film 801, a semiconductor film 803 positioned over the gateinsulating film 802 to overlap with the conductive film 801, andconductive films 804 and 805 that are positioned over the semiconductorfilm 803 and function as a source terminal and a drain terminal. Theconductive film 801 also functions as the scan line GLa. The conductivefilm 804 also functions as the signal line SL.

The transistor 34 includes, over the substrate 800 having an insulatingsurface, a conductive film 806 functioning as a gate electrode, the gateinsulating film 802 over the conductive film 806, a semiconductor film807 positioned over the gate insulating film 802 to overlap with theconductive film 806, and the conductive film 805 and a conductive film808 that are positioned over the semiconductor film 807 and function asa source terminal and a drain terminal. The conductive film 806 alsofunctions as the scan line GLb.

The transistor 33 includes, over the substrate 800 having an insulatingsurface, the conductive film 806 functioning as a gate electrode, thegate insulating film 802 over the conductive film 806, a semiconductorfilm 809 positioned over the gate insulating film 802 to overlap withthe conductive film 806, and conductive films 810 and 811 that arepositioned over the semiconductor film 809 and function as a sourceterminal and a drain terminal. The conductive film 811 also functions asthe line VLa.

The transistor 32 includes, over the substrate 800 having an insulatingsurface, the conductive film 801 functioning as a gate electrode, thegate insulating film 802 over the conductive film 801, a semiconductorfilm 812 positioned over the gate insulating film 802 to overlap withthe conductive film 801, and the conductive films 811 and 808 that arepositioned over the semiconductor film 812 and function as a sourceterminal and a drain terminal.

The transistor 31 includes, over the substrate 800 having an insulatingsurface, the conductive film 801 functioning as a gate electrode, thegate insulating film 802 over the conductive film 801, a semiconductorfilm 813 positioned over the gate insulating film 802 to overlap withthe conductive film 801, and the conductive film 810 and a conductivefilm 814 that are positioned over the semiconductor film 813 andfunction as a source terminal and a drain terminal. Note that theconductive film 814 is connected to the conductive film 805 through aconductive film 815.

The transistor 35 includes, over the substrate 800 having an insulatingsurface, the conductive film 806 functioning as a gate electrode, thegate insulating film 802 over the conductive film 806, a semiconductorfilm 816 positioned over the gate insulating film 802 to overlap withthe conductive film 806, and conductive films 817 and 818 that arepositioned over the semiconductor film 816 and function as a sourceterminal and a drain terminal.

The transistor 36 includes, over the substrate 800 having an insulatingsurface, a conductive film 823 functioning as a gate electrode, the gateinsulating film 802 over the conductive film 823, a semiconductor film824 positioned over the gate insulating film 802 to overlap with theconductive film 823, and the conductive film 818 and a conductive film825 that are positioned over the semiconductor film 824 and function asa source terminal and a drain terminal. Note that the conductive film823 also functions as the scan line GLc. The conductive film 825 isconnected to a conductive film 830 functioning as the line VLb.

The transistor 37 includes, over the substrate 800 having an insulatingsurface, the conductive film 823 functioning as a gate electrode, thegate insulating film 802 over the conductive film 823, a semiconductorfilm 826 positioned over the gate insulating film 802 to overlap withthe conductive film 823, and conductive films 827 and 828 that arepositioned over the semiconductor film 826 and function as a sourceterminal and a drain terminal. The conductive film 828 is connected to aconductive film 829 functioning as the line VLc.

The transistor 15 includes, over the substrate 800 having an insulatingsurface, a conductive film 831 functioning as a gate electrode, the gateinsulating film 802 over the conductive film 831, a semiconductor film832 positioned over the gate insulating film 802 to overlap with theconductive film 831, and the conductive films 810 and 818 that arepositioned over the semiconductor film 832 and function as a sourceterminal and a drain terminal. The transistor 15 further includesinsulating films 820 and 821 that are sequentially stacked over theconductive films 810 and 818, and a conductive film 833 that functionsas a back gate electrode and is positioned over the insulating films 820and 821 to overlap with the semiconductor film 832. The conductive film833 is connected to the conductive film 827. Further, the conductivefilm 831 is connected to the conductive film 814.

The capacitor 17 includes, over the substrate 800 having an insulatingsurface, a conductive film 834, the gate insulating film 802 over theconductive film 834, and the conductive film 818 positioned over thegate insulating film 802 to overlap with the conductive film 834. Theconductive film 834 is connected to the conductive film 808.

The capacitor 38 includes, over the substrate 800 having an insulatingsurface, a conductive film 835, the gate insulating film 802 over theconductive film 835, and the conductive film 818 positioned over thegate insulating film 802 to overlap with the conductive film 835. Theconductive film 835 is connected to the conductive film 827.

In addition, a conductive film 819 functioning as an anode is formedover the insulating film 821. The conductive film 819 is connected tothe conductive film 817 through an opening 822 formed in the insulatingfilms 820 and 821.

In addition, an insulating film 836 having an opening where part of theconductive film 819 is exposed is provided over the conductive film 819and the insulating film 821. An EL layer 837 and a conductive film 838functioning as a cathode are sequentially stacked over the part of theconductive film 819 and the insulating film 836. A region where theconductive film 819, the EL layer 837, and the conductive film 838overlap with each other corresponds to the light-emitting element 14.

Note that in one embodiment of the present invention, the semiconductorfilm 803, 807, 809, 812, 813, 816, 824, 826, or 832 may contain anamorphous, microcrystalline, polycrystalline, or single crystalsemiconductor (e.g., silicon or germanium) or a wide bandgapsemiconductor (e.g., an oxide semiconductor).

For a light-emitting device including an amorphous silicon or oxidesemiconductor transistor, a glass substrate of the fifth generation(1200 mm wide×1300 mm long) or later can be used. Thus, such alight-emitting device has advantages of high productivity and low cost.However, amorphous silicon or oxide semiconductor transistors generallyhave the same polarity and easily become normally on. In one embodimentof the present invention, even when the transistor 15 for controllingsupply of current to the light-emitting element 14 is normally on, thetransistor 15 is normally off by shifting the threshold voltage of thetransistor 15, so that the threshold voltage can be acquired. Thus, evenin a light-emitting device including an amorphous silicon or oxidesemiconductor transistor, luminance unevenness can be reduced andhigh-quality images can be displayed.

Note that when the semiconductor film 803, 807, 809, 812, 813, 816, 824,826, or 832 contains an amorphous, microcrystalline, polycrystalline, orsingle crystal semiconductor (e.g., silicon or germanium), an impurityregion functioning as a source region or a drain region is formed byaddition of an impurity element imparting conductivity to thesemiconductor film. For example, an impurity region having n-typeconductivity can be formed by addition of phosphorus or arsenic to thesemiconductor film. Further, for example, an impurity region havingp-type conductivity can be formed by addition of boron to thesemiconductor film.

When the semiconductor film 803, 807, 809, 812, 813, 816, 824, 826, or832 contains an oxide semiconductor, a dopant may be added to thesemiconductor film to form an impurity region functioning as a sourceregion or a drain region. The dopant can be added by ion implantation. Arare gas such as helium, argon, or xenon; a Group 15 atom such asnitrogen, phosphorus, arsenic, or antimony; or the like can be used asthe dopant. For example, in the case where nitrogen is used as thedopant, the concentration of nitrogen atoms in the impurity region ispreferably 5×10¹⁹/cm³ or higher and 1×10²²/cm³ or lower.

Note that as a silicon semiconductor, any of the following can be used:amorphous silicon formed by sputtering or vapor phase growth such asplasma-enhanced CVD; polycrystalline silicon obtained by crystallizationof amorphous silicon by treatment such as laser annealing; singlecrystal silicon obtained by separation of a surface portion of a singlecrystal silicon wafer by implantation of hydrogen ions or the like intothe silicon wafer; and the like.

Note that an oxide semiconductor preferably contains at least indium(In) or zinc (Zn). In particular, the oxide semiconductor preferablycontains In and Zn. As a stabilizer for reducing variations inelectrical characteristics of a transistor including the oxidesemiconductor, the oxide semiconductor preferably contains gallium (Ga)in addition to In and Zn. Tin (Sn) is preferably contained as astabilizer. Hafnium (Hf) is preferably contained as a stabilizer.Aluminum (Al) is preferably contained as a stabilizer.

As another stabilizer, one or more kinds of lanthanoid such as lanthanum(La), cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium (Sm),europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium(Ho), erbium (Er), thulium (Tm), ytterbium (Yb), or lutetium (Lu) may becontained.

For example, indium oxide; tin oxide; zinc oxide; a binary metal oxidesuch as an In—Zn-based oxide, a Sn—Zn-based oxide, an Al—Zn-based oxide,a Zn—Mg-based oxide, a Sn—Mg-based oxide, an In—Mg-based oxide, or anIn—Ga-based oxide; a ternary metal oxide such as an In—Ga—Zn-based oxide(also referred to as IGZO), an In—Al—Zn-based oxide, an In—Sn—Zn-basedoxide, a Sn—Ga—Zn-based oxide, an Al—Ga—Zn-based oxide, a Sn—Al—Zn-basedoxide, an In—Hf—Zn-based oxide, an In—La—Zn-based oxide, anIn—Ce—Zn-based oxide, an In—Pr—Zn-based oxide, an In—Nd—Zn-based oxide,an In—Sm—Zn-based oxide, an In—Eu—Zn-based oxide, an In—Gd—Zn-basedoxide, an In—Tb—Zn-based oxide, an In—Dy—Zn-based oxide, anIn—Ho—Zn-based oxide, an In—Er—Zn-based oxide, an In—Tm—Zn-based oxide,an In—Yb—Zn-based oxide, or an In—Lu—Zn-based oxide; or a quaternarymetal oxide such as an In—Sn—Ga—Zn-based oxide, an In—Hf—Ga—Zn-basedoxide, an In—Al—Ga—Zn-based oxide, an In—Sn—Al—Zn-based oxide, anIn—Sn—Hf—Zn-based oxide, or an In—Hf—Al—Zn-based oxide can be used as anoxide semiconductor. The oxide semiconductor may contain silicon.

Note that, for example, an In—Ga—Zn-based oxide means an oxidecontaining In, Ga, and Zn, and there is no limitation on the ratio ofIn, Ga, and Zn. In addition, the In—Ga—Zn-based oxide may contain ametal element other than In, Ga, and Zn. The In—Ga—Zn-based oxide hassufficiently high resistance when there is no electric field andoff-state current can be sufficiently reduced. Further, with highmobility, the In—Ga—Zn-based oxide is suitable for a semiconductormaterial used in a semiconductor device.

For example, an In—Ga—Zn-based oxide with an atomic ratio ofIn:Ga:Zn=1:1:1 (=1/3:1/3:1/3) or In:Ga:Zn=2:2:1 (=2/5:2/5:1/5), or anoxide whose composition is in the neighborhood of the above compositioncan be used. Alternatively, an In—Sn—Zn-based oxide with an atomic ratioof In:Sn:Zn=1:1:1 (=1/3:1/3:1/3), In:Sn:Zn=2:1:3 (=1/3:1/6:1/2), orIn:Sn:Zn=2:1:5 (=1/4:1/8:5/8), or an oxide whose composition is in theneighborhood of the above composition is preferably used.

For example, with an In—Sn—Zn-based oxide, high mobility can becomparatively easily obtained. However, even with an In—Ga—Zn-basedoxide, mobility can be increased by lowering defect density in a bulk.

Note that a highly-purified oxide semiconductor (a purified oxidesemiconductor) obtained by reduction of impurities such as moisture orhydrogen that serve as electron donors (donors) and reduction of oxygenvacancies is an intrinsic (i-type) semiconductor or a substantiallyintrinsic semiconductor. Thus, a transistor including the oxidesemiconductor has extremely low off-state current. Further, the band gapof the oxide semiconductor is 2 eV or more, preferably 2.5 eV or more,more preferably 3 eV or more. With the use of an oxide semiconductorfilm that is highly purified by a sufficient decrease in concentrationof impurities such as moisture or hydrogen and reduction of oxygenvacancies, the off-state current of the transistor can be decreased.

Specifically, various experiments can prove low off-state current of atransistor including a highly-purified oxide semiconductor for asemiconductor film. For example, even when an element has a channelwidth of 1×10⁶ μm and a channel length of 10 μm, off-state current canbe lower than or equal to the measurement limit of a semiconductorparameter analyzer, i.e., lower than or equal to 1×10⁻¹³ A, at a voltage(drain voltage) between a source terminal and a drain terminal of 1 to10 V. In that case, it can be seen that off-state current standardizedon the channel width of the transistor is lower than or equal to 100zA/μm. In addition, a capacitor and a transistor were connected to eachother and off-state current was measured using a circuit in whichelectrical charge flowing to or from the capacitor is controlled by thetransistor. In the measurement, a highly-purified oxide semiconductorfilm was used for a channel formation region of the transistor, and theoff-state current of the transistor was measured from a change in theamount of electrical charge of the capacitor per unit hour. As a result,it can be seen that, in the case where the voltage between the sourceterminal and the drain terminal of the transistor is 3 V, a loweroff-state current of several tens of yoctoamperes per micrometer (yA/μm)is obtained. Accordingly, the transistor including the highly-purifiedoxide semiconductor film for a channel formation region has much loweroff-state current than a crystalline silicon transistor.

Note that unless otherwise specified, in this specification, off-statecurrent of an re-channel transistor is current that flows between asource terminal and a drain terminal when the potential of the drainterminal is higher than that of the source terminal or that of a gateelectrode while the potential of the gate electrode is 0 V or lower inthe case of the potential of the source terminal used as a reference.Alternatively, in this specification, off-state current of a p-channeltransistor is current that flows between a source terminal and a drainterminal when the potential of the drain terminal is lower than that ofthe source terminal or that of a gate electrode while the potential ofthe gate electrode is 0 V or higher in the case of the potential of thesource terminal used as a reference.

For example, the oxide semiconductor film can be formed by sputteringusing a target including indium (In), gallium (Ga), and zinc (Zn). Inthe case where an In—Ga—Zn-based oxide semiconductor film is formed bysputtering, it is preferable to use a target of an In—Ga—Zn-based oxidewith an atomic ratio of In:Ga:Zn=1:1:1, 4:2:3, 3:1:2, 1:1:2, 2:1:3, or3:1:4. When an oxide semiconductor film is formed using a target of anIn—Ga—Zn-based oxide having the above atomic ratio, a polycrystal or ac-axis-aligned crystal (CAAC) OS to be described later is easily formed.The filling factor of the target including In, Ga, and Zn is higher thanor equal to 90% and lower than or equal to 100%, preferably higher thanor equal to 95% and lower than 100%. With the use of the target with ahigh filling factor, a dense oxide semiconductor film is formed.

In the case where an In—Zn-based material is used for the oxidesemiconductor, a target used has an atomic ratio of In:Zn=50:1 to 1:2(In₂O₃:ZnO=25:1 to 1:4 in a mole ratio), preferably In:Zn=20:1 to 1:1(In₂O₃:ZnO=10:1 to 1:2 in a mole ratio), more preferably In:Zn=1.5:1 to15:1 (In₂O₃:ZnO=3:4 to 15:2 in a mole ratio). For example, when a targetused for deposition of an oxide semiconductor film formed using anIn—Zn-based oxide has an atomic ratio of In:Zn:O=X:Y:Z, Z>1.5X+Y. Themobility can be increased by keeping the ratio of Zn within the aboverange.

Specifically, the oxide semiconductor film may be deposited in such amanner that the substrate is held in a treatment chamber kept in areduced pressure state, moisture remaining in the treatment chamber isremoved, a sputtering gas from which hydrogen and moisture are removedis introduced, and the target is used. The substrate temperature may be100 to 600° C., preferably 200 to 400° C. during deposition. Bydeposition of the oxide semiconductor film while the substrate isheated, the concentration of impurities included in the deposited oxidesemiconductor film can be lowered. In addition, damage by sputtering canbe reduced. In order to remove moisture remaining in the treatmentchamber, an adsorption vacuum pump is preferably used. For example, acryopump, an ion pump, or a titanium sublimation pump is preferablyused. A turbo pump to which a cold trap is added may be used as anexhaustion means. For example, a hydrogen atom, a compound containing ahydrogen atom, such as water (preferably a compound containing a carbonatom), and the like are exhausted from the treatment chamber with theuse of a cryopump. Thus, the concentration of impurities contained inthe oxide semiconductor film deposited in the treatment chamber can belowered.

Note that the oxide semiconductor film formed by sputtering or the likecontains a large amount of moisture or hydrogen (including a hydroxylgroup) as an impurity in some cases. Moisture and hydrogen easily formdonor levels and thus serve as impurities in the oxide semiconductor.Thus, in one embodiment of the present invention, in order to reduceimpurities such as moisture or hydrogen in the oxide semiconductor film(in order to perform dehydration or dehydrogenation), the oxidesemiconductor film is subjected to heat treatment in a reduced-pressureatmosphere, an inert gas atmosphere of nitrogen, a rare gas, or thelike, an oxygen gas atmosphere, or ultra dry air (the moisture amount is20 ppm (−55° C. by conversion into a dew point) or less, preferably 1ppm or less, more preferably 10 ppb or less, in the case wheremeasurement is performed by a dew point meter in a cavity ring-downlaser spectroscopy (CRDS) method).

By performing heat treatment on the oxide semiconductor film, moistureor hydrogen in the oxide semiconductor film can be eliminated.Specifically, heat treatment may be performed at a temperature higherthan or equal to 250° C. and lower than or equal to 750° C., preferablyhigher than or equal to 400° C. and lower than the strain point of thesubstrate. For example, heat treatment may be performed at 500° C. forapproximately 3 to 6 minutes. When RTA is used for the heat treatment,dehydration or dehydrogenation can be performed in a short time; thus,treatment can be performed even at a temperature higher than the strainpoint of a glass substrate.

Note that in some cases, the heat treatment makes oxygen released fromthe oxide semiconductor film and oxygen vacancies occur in the oxidesemiconductor film. Thus, in one embodiment of the present invention, aninsulating film containing oxygen is used as an insulating film that isin contact with the oxide semiconductor film, such as a gate insulatingfilm. Then, heat treatment is performed after formation of theinsulating film containing oxygen, so that oxygen is supplied from theinsulating film to the oxide semiconductor film. With this structure,oxygen vacancies that serve as donors can be reduced and thestoichiometric proportion of the oxide semiconductor included in theoxide semiconductor film can be satisfied. It is preferable that theproportion of oxygen in the oxide semiconductor film be higher than thestoichiometric proportion. As a result, the oxide semiconductor film canbe substantially intrinsic and variations in electrical characteristicsof the transistor due to oxygen vacancies can be reduced, which resultsin an improvement of electrical characteristics.

Note that the heat treatment for supplying oxygen to the oxidesemiconductor film is performed in an atmosphere of nitrogen, ultra dryair, or a rare gas (e.g., argon or helium) preferably at 200 to 400° C.,for example, 250 to 350° C. It is preferable that the water content inthe gas be 20 ppm or less, preferably 1 ppm or less, more preferably 10ppb or less.

The oxide semiconductor may be either amorphous or crystalline. In thelatter case, the oxide semiconductor may be either single crystalline orpolycrystalline, may have a structure in which part of the oxidesemiconductor is crystalline, may have an amorphous structure includinga crystalline portion, or may be non-amorphous. As an example of thestructure in which part of the oxide semiconductor is crystalline, anoxide semiconductor including a crystal with c-axis alignment (alsoreferred to as a c-axis aligned crystalline oxide semiconductor(CAAC-OS)) that has a triangular or hexagonal atomic order when seenfrom the direction perpendicular to the a-b plane, a surface, or aninterface may be used. In the crystal, metal atoms are arranged in alayered manner, or metal atoms and oxygen atoms are arranged in alayered manner when seen from the direction perpendicular to the c-axis,and the direction of the a-axis or the b-axis is varied in the a-b plane(the crystal rotates around the c-axis).

In a broad sense, CAAC-OS means a non-single-crystal oxide including aphase that has a triangular, hexagonal, regular triangular, or regularhexagonal atomic order when seen from the direction perpendicular to thea-b plane and in which metal atoms are arranged in a layered manner ormetal atoms and oxygen atoms are arranged in a layered manner when seenfrom the direction perpendicular to the c-axis direction.

CAAC-OS is not single crystal but this does not mean that CAAC-OS iscomposed of only an amorphous component. Although CAAC-OS includes acrystal portion, a boundary between one crystal portion and anothercrystal portion is not clear in some cases.

Nitrogen may be substituted for part of oxygen included in CAAC-OS. Thec-axes of crystal portions included in CAAC-OS may be aligned in acertain direction (e.g., a direction perpendicular to a surface of asubstrate over which CAAC-OS is formed or a surface of CAAC-OS).Alternatively, the normals of the a-b planes of the crystal portionsincluded in CAAC-OS may be aligned in a certain direction (e.g., adirection perpendicular to a surface of a substrate over which CAAC-OSis formed or a surface of CAAC-OS).

CAAC-OS transmits or does not transmit visible light depending on itscomposition or the like.

As an example of such CAAC-OS, there is a crystal that is formed into afilm shape and has a triangular or hexagonal atomic order when seen fromthe direction perpendicular to a surface of the film or a surface of asubstrate over which CAAC-OS is formed, and in which metal atoms arearranged in a layered manner or metal atoms and oxygen atoms (ornitrogen atoms) are arranged in a layered manner when a cross section ofthe film is observed.

For example, a CAAC-OS film is deposited by sputtering with apolycrystalline oxide semiconductor sputtering target. When ions collidewith the sputtering target, a crystal region included in the sputteringtarget may be separated from the target along the a-b plane, and asputtered particle having a plane parallel to the a-b plane (aflat-plate-like sputtered particle or a pellet-like sputtered particle)might be separated from the sputtering target. In that case, theflat-plate-like sputtered particle reaches a substrate while maintainingits crystal state, so that the CAAC-OS film can be deposited.

For the deposition of the CAAC-OS film, the following conditions arepreferably employed.

By reducing the amount of impurities entering the CAAC-OS film duringthe deposition, the crystal state can be prevented from being broken bythe impurities. For example, the concentration of impurities (e.g.,hydrogen, water, carbon dioxide, or nitrogen) which exist in thetreatment chamber may be reduced. Further, the concentration ofimpurities in a deposition gas may be reduced. Specifically, adeposition gas whose dew point is −80° C. or lower, preferably −100° C.or lower is used.

By increasing the substrate heating temperature during the deposition,migration of a sputtered particle occurs after the sputtered particlereaches the substrate. Specifically, the substrate heating temperatureduring the deposition is 100° C. or higher and 740° C. or lower,preferably 200° C. or higher and 500° C. or lower. By increasing thesubstrate heating temperature during the deposition, when theflat-plate-like sputtered particle reaches the substrate, migrationoccurs on the substrate, so that a flat plane of the sputtered particleis attached to the substrate.

Further, it is preferable to reduce plasma damage during the depositionby increasing the proportion of oxygen in the deposition gas andoptimizing power. The proportion of oxygen in the deposition gas is 30vol % or higher, preferably 100 vol %.

As an example of the sputtering target, an In—Ga—Zn—O compound target isdescribed below.

A polycrystalline In—Ga—Zn—O compound target is made by mixing InO_(X)powder, GaO_(Y) powder, and ZnO_(Z) powder in a predetermined moleratio, applying pressure, and performing heat treatment at 1000° C. orhigher and 1500° C. or lower. Note that X, Y, and Z are each a givenpositive number. Here, the predetermined mole ratio of the InO_(X)powder, the GaO_(Y) powder, and the ZnO_(Z) powder is, for example,2:2:1, 8:4:3, 3:1:1, 1:1:1, 4:2:3, or 3:1:2. The kinds of powder and themole ratio for mixing powder may be changed as appropriate depending ona sputtering target to be formed.

This embodiment can be combined with any of the other embodiments asappropriate.

Embodiment 5

In a light-emitting device according to one embodiment of the presentinvention, it is possible to employ a color filter method in whichfull-color images are displayed using a combination of a color filterand a light-emitting element that emits light of a single color such aswhite. Alternatively, it is possible to employ a method in whichfull-color images are displayed using a plurality of light-emittingelements that emit light of different hues. This method is referred toas a separate coloring method because EL layers each provided between apair of electrodes of a light-emitting element are separately coloredwith corresponding colors.

In the separate coloring method, in general, EL layers are separatelyapplied by vapor deposition with the use of a mask such as a metal mask.Thus, the size of pixels depends on the accuracy of separate coloring ofthe EL layers by vapor deposition. On the other hand, unlike theseparate coloring method, EL layers do not need to be separately appliedin the color filter method. Accordingly, pixels can be downsized moreeasily as compared to the separate coloring method; thus, ahigh-definition pixel portion can be provided.

A light-emitting device includes, in its category, a bottom-emissionlight-emitting device in which light emitted from a light-emittingelement is extracted from an element substrate over which a transistoris formed, and a top-emission light-emitting device in which lightemitted from a light-emitting element is extracted from a side oppositeto an element substrate. In the top-emission light-emitting device,light emitted from a light-emitting element is not blocked by an elementsuch as a wiring, a transistor, or a capacitor, so that the efficiencyof light extraction from a pixel can be made higher than that in thebottom-emission light-emitting device. Thus, the top-emissionlight-emitting device can achieve high luminance even when the value ofcurrent supplied to a light-emitting element is decreased, and thus isadvantageous in improving the lifetime of the light-emitting element.

The light-emitting device according to one embodiment of the presentinvention may have a microcavity (micro optical resonator) structure inwhich light emitted from an EL layer resonates in a light-emittingelement. With the microcavity structure, light having a specificwavelength can be extracted from the light-emitting element with highefficiency, so that the luminance and color purity of the pixel portioncan be improved.

FIG. 13 is an example of a cross-sectional view of pixels. Note thatFIG. 13 illustrates part of a cross section of a pixel corresponding tored, part of a cross section of a pixel corresponding to green, and partof a cross section of a pixel corresponding to blue.

Specifically, FIG. 13 illustrates a pixel 140 r corresponding to red, apixel 140 g corresponding to green, and a pixel 140 b corresponding toblue. The pixel 140 r, the pixel 140 g, and the pixel 140 b include ananode 715 r, an anode 715 g, and an anode 715 b, respectively. Theanodes 715 r, 715 g, and 715 b included in the pixels 140 r, 140 g, and140 b are provided over an insulating film 750 formed over a substrate740.

A bank 730 formed using an insulating film is provided over the anodes715 r, 715 g, and 715 b. The bank 730 has openings, where parts of theanodes 715 r, 715 g, and 715 b are exposed. An EL layer 731 and acathode 732 that transmits visible light are stacked in that order overthe bank 730 to cover the exposed parts.

A portion where the anode 715 r, the EL layer 731, and the cathode 732overlap with each other corresponds to a light-emitting element 741 rcorresponding to red. A portion where the anode 715 g, the EL layer 731,and the cathode 732 overlap with each other corresponds to alight-emitting element 741 g corresponding to green. A portion where theanode 715 b, the EL layer 731, and the cathode 732 overlap with eachother corresponds to a light-emitting element 741 b corresponding toblue.

A substrate 742 faces the substrate 740 with the light-emitting elements741 r, 741 g, and 741 b placed therebetween. A coloring layer 743 rcorresponding to the pixel 140 r, a coloring layer 743 g correspondingto the pixel 140 g, and a coloring layer 743 b corresponding to thepixel 140 b are provided on the substrate 742. The coloring layer 743 ris a layer whose transmittance of light in a wavelength rangecorresponding to red is higher than that of light in other wavelengthranges. The coloring layer 743 g is a layer whose transmittance of lightin a wavelength range corresponding to green is higher than that oflight in other wavelength ranges. The coloring layer 743 b is a layerwhose transmittance of light in a wavelength range corresponding to blueis higher than that of light in other wavelength ranges.

An overcoat 744 is provided on the substrate 742 to cover the coloringlayers 743 r, 743 g, and 743 b. The overcoat 744 transmits visiblelight, is provided for protecting the coloring layers 743 r, 743 g, and743 b, and is preferably formed using a highly flattened resin material.The coloring layers 743 r, 743 g, and 743 b and the overcoat 744 may becollectively regarded as a color filter, or each of the coloring layers743 r, 743 g, and 743 b may be regarded as a color filter.

In FIG. 13, a conductive film 745 r with high visible-light reflectanceand a conductive film 746 r with higher visible-light transmittance thanthe conductive film 745 r are stacked in that order as the anode 715 r.A conductive film 745 g with high visible-light reflectance and aconductive film 746 g with higher visible-light transmittance than theconductive film 745 g are stacked in that order as the anode 715 g. Theconductive film 746 g is thinner than the conductive film 746 r. Aconductive film 745 b with high visible-light reflectance is used as theanode 715 b.

Thus, in the light-emitting device in FIG. 13, the optical path lengthof light emitted from the EL layer 731 in the light-emitting element 741r can be adjusted by the distance between the conductive film 745 r andthe cathode 732. The optical path length of light emitted from the ELlayer 731 in the light-emitting element 741 g can be adjusted by thedistance between the conductive film 745 g and the cathode 732. Theoptical path length of light emitted from the EL layer 731 in thelight-emitting element 741 b can be adjusted by the distance between theconductive film 745 b and the cathode 732.

In one embodiment of the present invention, a microcavity structure maybe employed in which the optical path lengths are adjusted in accordancewith the wavelengths of light emitted from the light-emitting elements741 r, 741 g, and 741 b so that light emitted from the EL layer 731resonates in each light-emitting element.

When the microcavity structure is applied to the light-emitting deviceaccording to one embodiment of the present invention, light with awavelength corresponding to red among the light emitted from thelight-emitting element 741 r resonates in the microcavity structure toincrease its intensity. Consequently, the color purity and luminance ofred light obtained through the coloring layer 743 r are improved. Lightwith a wavelength corresponding to green among the light emitted fromthe light-emitting element 741 g resonates in the microcavity structureto increase its intensity. Consequently, the color purity and luminanceof green light obtained through the coloring layer 743 g are improved.Light with a wavelength corresponding to blue among the light emittedfrom the light-emitting element 741 b resonates in the microcavitystructure to increase its intensity. Consequently, the color purity andluminance of blue light obtained through the coloring layer 743 b areimproved.

Note that although the pixels corresponding to three colors of red,green, and blue are shown in FIG. 13, one embodiment of the presentinvention is not limited to this structure. In one embodiment of thepresent invention, a combination of four colors of red, green, blue, andyellow or a combination of three colors of cyan, magenta, and yellow maybe used. Alternatively, a combination of six colors of pale red, palegreen, pale blue, deep red, deep green, and deep blue, or a combinationof six colors of red, green, blue, cyan, magenta, and yellow may beused.

Note that colors that can be expressed using pixels of red, green, andblue, for example, are limited to colors existing in a triangle made bythree points on a chromaticity diagram that correspond to the emissioncolors of the pixels. Thus, by additionally providing a light-emittingelement of a color existing outside the triangle on the chromaticitydiagram as in the case where pixels of red, green, blue, and yellow areused, the range of the colors that can be expressed in thelight-emitting device can be expanded and the color reproducibility canbe improved.

In FIG. 13, the conductive film 745 b with high visible-lightreflectance is used as the anode in the light-emitting element 741 bwhich emits light with the shortest wavelength λ among thelight-emitting elements 741 r, 741 g, and 741 b, and the conductivefilms 746 r and 746 g having different thicknesses are used in the otherlight-emitting elements 741 r and 741 g; thus, the optical path lengthsare adjusted. In one embodiment of the present invention, a conductivefilm with high visible-light transmittance, such as the conductive films746 r and 746 g, may be provided over the conductive film 745 b withhigh visible-light reflectance also in the light-emitting element 741 bwhich emits light with the shortest wavelength λ. However, it ispreferable to use the conductive film 745 b with high visible-lightreflectance as the anode of the light-emitting element 741 b which emitslight with the shortest wavelength λ as shown in FIG. 13, because thefabrication process of the anode can be simplified as compared to thecase where a conductive film with high visible-light transmittance isused as the anodes of all the light-emitting elements.

Note that the work function of the conductive film 745 b with highvisible-light reflectance is often lower than those of the conductivefilms 746 r and 746 g with high visible-light transmittance.Accordingly, in the light-emitting element 741 b which emits light withthe shortest wavelength λ, holes are less likely to be injected from theanode 715 b into the EL layer 731 than in the light-emitting elements741 r and 741 g, resulting in low emission efficiency. In view of this,in one embodiment of the present invention, a composite material thatcontains a substance having a high hole-transport property and asubstance having an acceptor property (electron-accepting property) withrespect to the substance having a high hole-transport property ispreferably used for part of the EL layer 731 that is in contact with theconductive film 745 b with high visible-light reflectance in thelight-emitting element 741 b which emits light with the shortestwavelength λ. When the composite material is formed in contact with theanode 715 b, holes can be easily injected from the anode 715 b into theEL layer 731, so that the emission efficiency of the light-emittingelement 741 b can be increased.

Examples of the substance having an acceptor property are7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethane (F₄-TCNQ),chloranil, a transition metal oxide, and oxides of metals that belong toGroups 4 to 8 in the periodic table. Specifically, vanadium oxide,niobium oxide, tantalum oxide, chromium oxide, molybdenum oxide,tungsten oxide, manganese oxide, and rhenium oxide are preferablebecause of their high acceptor properties. Among these, molybdenum oxideis particularly preferable because it is stable in the air, has a lowhygroscopic property, and is easily treated.

As the substance having a high hole-transport property used for thecomposite material, any of a variety of compounds such as an aromaticamine compound, a carbazole derivative, aromatic hydrocarbon, and a highmolecular compound (e.g., an oligomer, a dendrimer, or a polymer) can beused. An organic compound used for the composite material is preferablyan organic compound having a high hole-transport property. Specifically,a substance having a hole mobility of 10⁻⁶ cm²/V·s or higher ispreferably used. Note that any other substance may be used as long asits hole-transport property is higher than its electron-transportproperty.

The conductive films 745 r, 745 g, and 745 b having high visible-lightreflectance can be formed with a single layer or a stack of aluminum,silver, or an alloy containing such a metal material, for example.Alternatively, the conductive films 745 r, 745 g, and 745 b may beformed by stacking a conductive film with high visible-light reflectanceand a thin conductive film (preferably with a thickness of 20 nm orless, more preferably 10 nm or less). For example, a thin titanium filmor a thin molybdenum film is stacked over a conductive film with highvisible-light reflectance to form the conductive film 745 b, so that anoxide film can be prevented from being formed on a surface of theconductive film with high visible-light reflectance (e.g., aluminum, analloy containing aluminum, or silver).

The conductive films 746 r and 746 g with high visible-lighttransmittance can be formed using, for example, indium oxide, tin oxide,zinc oxide, indium tin oxide, or indium zinc oxide.

The cathode 732 can be formed, for example, by stacking a conductivefilm thin enough to transmit light (preferably with a thickness of 20 nmor less, more preferably 10 nm or less) and a conductive film includinga conductive metal oxide. The conductive film thin enough to transmitlight can be formed with a single layer or a stack of silver, magnesium,an alloy containing such a metal material, or the like. Examples of theconductive metal oxide are indium oxide, tin oxide, zinc oxide, indiumtin oxide, indium zinc oxide, and any of these metal oxide materialscontaining silicon oxide.

This embodiment can be combined with any of the other embodiments asappropriate.

Embodiment 6

In this embodiment, a bottom-emission structure, a top-emissionstructure, and a dual-emission structure are described. In thedual-emission structure, light from a light-emitting element isextracted from an element substrate side and a side opposite to theelement substrate.

FIG. 14A is a cross-sectional view of a pixel in which light emittedfrom a light-emitting element 6033 is extracted from an anode 6034 side.A transistor 6031 is covered with an insulating film 6037, and a bank6038 having an opening is formed over the insulating film 6037. In theopening of the bank 6038, the anode 6034 is partly exposed, and theanode 6034, an EL layer 6035, and a cathode 6036 are stacked in thatorder in the opening.

The anode 6034 is formed using a material through which light passeseasily or formed to a thickness such that light passes through the anode6034 easily. The cathode 6036 is formed using a material through whichlight does not easily pass or formed to a thickness such that light doesnot easily pass through the cathode 6036. Accordingly, it is possible toobtain a bottom-emission structure in which light is extracted from theanode 6034 side as indicated by an outline arrow.

FIG. 14B is a cross-sectional view of a pixel in which light emittedfrom a light-emitting element 6043 is extracted from a cathode 6046side. A transistor 6041 is covered with an insulating film 6047, and abank 6048 having an opening is formed over the insulating film 6047. Inthe opening of the bank 6048, an anode 6044 is partly exposed, and theanode 6044, an EL layer 6045, and the cathode 6046 are stacked in thatorder in the opening.

The anode 6044 is formed using a material through which light does noteasily pass or formed to a thickness such that light does not easilypass through the anode 6044. The cathode 6046 is formed using a materialthrough which light passes easily or formed to a thickness such thatlight passes through the cathode 6046 easily. Accordingly, it ispossible to obtain a top-emission structure in which light is extractedfrom the cathode 6046 side as indicated by an outline arrow.

FIG. 14C is a cross-sectional view of a pixel in which light emittedfrom a light-emitting element 6053 is extracted from an anode 6054 sideand a cathode 6056 side. A transistor 6051 is covered with an insulatingfilm 6057, and a bank 6058 having an opening is formed over theinsulating film 6057. In the opening of the bank 6058, the anode 6054 ispartly exposed, and the anode 6054, an EL layer 6055, and the cathode6056 are stacked in that order in the opening.

The anode 6054 and the cathode 6056 are formed using a material throughwhich light passes easily or formed to a thickness such that lightpasses through the anode 6054 and the cathode 6056 easily. Accordingly,it is possible to obtain a dual-emission structure in which light isextracted from the anode 6054 side and the cathode 6056 side asindicated by outline arrows.

For the electrode serving as the anode or the cathode, any of metals,alloys, electrically conductive compounds, and mixtures thereof can beused, for example. Specific examples are indium oxide-tin oxide (indiumtin oxide (ITO)), indium oxide-tin oxide containing silicon or siliconoxide, indium oxide-zinc oxide (indium zinc oxide), indium oxidecontaining tungsten oxide and zinc oxide, gold (Au), platinum (Pt),nickel (Ni), tungsten (W), chromium (Cr), molybdenum (Mo), iron (Fe),cobalt (Co), copper (Cu), palladium (Pd), and titanium (Ti). Otherexamples are elements that belong to Group 1 or 2 in the periodic table,for example, an alkali metal such as lithium (Li) or cesium (Cs), analkaline earth metal such as calcium (Ca) or strontium (Sr), magnesium(Mg), an alloy containing such an element (e.g., MgAg or AlLi), a rareearth metal such as europium (Eu) or ytterbium (Yb), an alloy containingsuch an element, and graphene. The electrode is formed using a materialselected from the above as appropriate and formed to an optimumthickness, so that a top-emission structure, a bottom-emissionstructure, or a dual-emission structure can be selectively formed.

This embodiment can be combined with any of the other embodiments asappropriate.

Embodiment 7

FIG. 15 is an example of a perspective view of a light-emitting deviceaccording to one embodiment of the present invention.

The light-emitting device in FIG. 15 includes a panel 1601, a circuitboard 1602, and joints 1603. The panel 1601 includes a pixel portion1604 including a plurality of pixels, a scan line driver circuit 1605that selects a plurality of pixels row by row, and a signal line drivercircuit 1606 that controls input of image signals to the pixels in aselected row. Specifically, signals to be input to a variety of scanlines are generated in the scan line driver circuit 1605.

A variety of signals and power supply potentials are input from thecircuit board 1602 to the panel 1601 through the joints 1603. A flexibleprinted circuit (FPC) or the like can be used as the joint 1603. In thecase where a COF tape is used as the joint 1603, part of circuits in thecircuit board 1602 or part of the scan line driver circuit 1605 or thesignal line driver circuit 1606 included in the panel 1601 may be formedon a chip separately prepared, and the chip may be connected to the COFtape by chip on film (COF).

This embodiment can be combined with any of the other embodiments.

Embodiment 8

A light-emitting device according to one embodiment of the presentinvention can be used for display devices, personal computers, or imagereproducing devices provided with recording media (typically, devicesthat reproduce the content of recording media such as digital versatilediscs (DVDs) and have displays for displaying the reproduced images).Further, as electronic devices that can include the light-emittingdevice according to one embodiment of the present invention, cellularphones, game machines (including portable game machines), personaldigital assistants, e-book readers, cameras such as video cameras anddigital still cameras, goggle-type displays (head mounted displays),navigation systems, audio reproducing devices (e.g., car audio systemsand digital audio players), copiers, facsimiles, printers, multifunctionprinters, automated teller machines (ATMs), vending machines, and thelike can be given. FIGS. 16A to 16E illustrate specific examples ofthese electronic devices.

FIG. 16A illustrates a portable game machine, which includes a housing5001, a housing 5002, a display portion 5003, a display portion 5004, amicrophone 5005, speakers 5006, an operation key 5007, a stylus 5008,and the like. It is possible to provide a high-definition portable gamemachine having less luminance unevenness with the use of thelight-emitting device according to one embodiment of the presentinvention as the display portion 5003 or 5004. Note that although theportable game machine in FIG. 16A has the two display portions 5003 and5004, the number of display portions included in the portable gamemachine is not limited thereto.

FIG. 16B illustrates a display device, which includes a housing 5201, adisplay portion 5202, a support 5203, and the like. It is possible toprovide a high-definition display device having less luminanceunevenness with the use of the light-emitting device according to oneembodiment of the present invention as the display portion 5202. Notethat the display device means all display devices for displayinginformation, such as display devices for personal computers, forreceiving TV broadcast, and for displaying advertisements.

FIG. 16C illustrates a laptop, which includes a housing 5401, a displayportion 5402, a keyboard 5403, a pointing device 5404, and the like. Itis possible to provide a high-definition laptop having less luminanceunevenness with the use of the light-emitting device according to oneembodiment of the present invention as the display portion 5402.

FIG. 16D illustrates a personal digital assistant, which includes afirst housing 5601, a second housing 5602, a first display portion 5603,a second display portion 5604, a joint 5605, an operation key 5606, andthe like. The first display portion 5603 is provided in the firsthousing 5601, and the second display portion 5604 is provided in thesecond housing 5602. The first housing 5601 and the second housing 5602are connected to each other with the joint 5605, and an angle betweenthe first housing 5601 and the second housing 5602 can be changed withthe joint 5605. An image on the first display portion 5603 may beswitched depending on the angle between the first housing 5601 and thesecond housing 5602 at the joint 5605. It is possible to provide ahigh-definition personal digital assistant having less luminanceunevenness with the use of the light-emitting device according to oneembodiment of the present invention as the first display portion 5603 orthe second display portion 5604. A light-emitting device with a positioninput function may be used as at least one of the first display portion5603 and the second display portion 5604. Note that the position inputfunction can be added by provision of a touch panel in a light-emittingdevice. Alternatively, the position input function can be added byprovision of a photoelectric conversion element called a photosensor ina pixel portion of a light-emitting device.

FIG. 16E illustrates a cellular phone, which includes a housing 5801, adisplay portion 5802, an audio input portion 5803, an audio outputportion 5804, operation keys 5805, a light receiving portion 5806, andthe like. Light received in the light receiving portion 5806 isconverted into electrical signals, so that external images can beloaded. It is possible to provide a high-definition cellular phonehaving less luminance unevenness with the use of the light-emittingdevice according to one embodiment of the present invention as thedisplay portion 5802.

This embodiment can be combined with any of the other embodiments asappropriate.

This application is based on Japanese Patent Application serial No.2011-200067 filed with Japan Patent Office on Sep. 14, 2011, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A light-emitting device comprising: a light-emitting element; a transistor; a first switch; a second switch; and a third switch, wherein the transistor controls supply of current to the light-emitting element and comprises a semiconductor film and a first gate electrode and a second gate electrode facing each other with the semiconductor film provided therebetween, wherein the first switch controls supply of a potential of an image signal to the first gate electrode of the transistor, wherein the second switch controls supply of a potential to the second gate electrode of the transistor, and wherein the third switch controls connection between the first gate electrode and a drain terminal of the transistor. 